The present invention relates generally to photomasks used for producing high-density integrated circuits such as LSIs and VLSIs and more specifically to phase shift layer-containing photomasks used for forming fine patterns with high accuracy as well as their production and correction.
So far, semiconductor integrated circuits such as ICs, LSIs and VLSIs have been produced by repeating a so-called lithographic cycle wherein resists are applied on substrates to be processed, like Si wafers, and the substrates are then exposed to light through steppers to form the desired patterns, followed by development and etching.
The photomasks used in such a lithographic step and called "reticles" are now increasingly required to have much higher accuracy as current semiconductor integrated circuits are higher in performance and integration than ever before. Referring to a DRAM that is a typical LSI as an example, a dimensional variation of a five-fold reticle for a 1 megabit DRAM, i.e., of a reticle five-fold greater in size than the pattern to be exposed to light is required to have an accuracy of 0.15 .mu.m even where the mean value =.+-.3.delta. (.delta. is standard deviation). Likewise, five-fold reticles for 4- and 16-megabit DRAMs are required to have an accuracy of 0.1-0.15 .mu.m and 0.05-0.1 .mu.m, respectively.
Furthermore, the line widths of device patterns formed with these reticles must be much finer, say, 1.2 .mu.m for 1-megabit DRAMs and 0.8 .mu.m for 16-megabit DRAMs, and various exposure processes are now being studied to meet such demand.
With the next, . . . generation device patterns of, e.g. the 64 megabit DRAM class, however, stepper exposure systems using conventional reticles have been found to place some limitation on the resolution of resist patterns. Thus, a version of reticle based on a new idea, like those set forth in Japanese Provisional Patent Publication No. 58(1983)-173744, Japanese Patent Publication No. 62(1987)-59296, etc. and referred to as phase shift masks, has been proposed in the art. Phase shift lithography using this reticle is a technique designed to improve the resolving power and contrast of projected images by operating the phase of light transmitting through the reticle.
Phase shift lithography will now be explained with reference to FIGS. 2a to 2d and FIGS. 3a to 3d. FIGS. 2a to 2d are diagrammatical sketches showing the principium of the phase shift process and FIGS. 3a to 3d are diagrammatical sketches illustrating a conventional process. FIGS. 2a and 3a are sectional views of reticles; FIGS. 2b and 3b show the amplitude of light on the reticles; FIGS. 2c and 3c depict the amplitude of light on wafers; and FIGS. 2d and 3d illustrate the intensity of light on the wafers. In FIGS. 2 or 3, reference numeral 1 stands for a substrate, 2 a light-shielding film, 3 a phase shifter and 4 incident light.
In the conventional process, as illustrated in FIG. 3a, the substrate 1 formed of glass, etc. is provided thereon with the light-shielding film 2 formed of chromium, etc., only to form a given pattern of light-transmitting regions. In phase shift lithography, however, the phase shifter 3 to cause phase reversal (with a phase difference of 180.degree.) is mounted on one pair of adjacent light-transmitting regions on a reticle, as sketched in FIG. 2a. According to the conventional process, therefore, the amplitude of light on the reticle is in the same phase, as illustrated in FIG. 3b, as is the amplitude of light on the wafer, as depicted in FIG. 3c, with the result that the patterns on the wafer cannot be separated from each other, as sketched in FIG. 3d. By contrast, the phase shift lithography enables the adjacent patterns to be distinctly separated from each other, as illustrated in FIG. 2d, because the light transmitting through the phase shifter is reversed in phase between the adjacent patterns, as depicted in FIG. 2b, so that the intensity of light on the pattern boundary can be reduced to zero. With the phase shift lithography, even patterns which cannot heretofore be separated from each other are thus made separable from each other, thereby achieving high resolution..
Then, one example of conventional processes of producing phase shift reticles will be explained with reference to FIGS. 14a to 14m which are a series of sectional views illustrating the steps of producing a typical phase shift reticle. As illustrated in FIG. 14a, a chromium film 12 is first formed on an optically polished substrate 11, and an ionizing radiation resist is uniformly coated and heated for drying thereon in conventional-manners to form a resist layer 13. Then, a pattern is drawn on the resist layer 13 with ionizing radiations 14, as shown in FIG. 14b, followed by development and rinsing, thereby forming such a resist pattern 15 as sketched in FIG. 14c.
If required, the edges, etc. of the resist pattern 15 are then cleared of unnecessary matter such as resist scums and whiskers by heating and descumming. Afterwards, portions of the chronium film exposed to open view between the resist pattern lines 15 are dry-etched using an etching gas plasma 16, as shown in FIG. 14d, thereby forming a chromium pattern 17. After etching in this manner, the resist pattern 15, viz., the remaining resist is incinerated out by an oxygen plasma 18, as shown in FIG. 14e, thereby obtaining such a complete photomask as shown in FIG. 14f.
Subsequently, this photomask is inspected to make a correction on the pattern, if required, followed by washing. After that, a transparent film 19 formed of, e.g. SiO.sub.2 is formed on the chromium pattern 17, as depicted in FIG. 14g. Then, as depicted in FIG. 14h, an ionizing radiation resist 20 is formed on the transparent film 19 in similar manners as mentioned above, followed by alignment of the resist patterns 20, as shown in FIG. 14i. Subsequent drawing of a given pattern with ionizing radiations 21, development and rinsing give a resist pattern, as illustrated in FIG. 14j. Then, heating and descumming are effected, if required. After that, portions of the transparent film 19 exposed to open view between the resist pattern lines 22 are dry-etched by means of an etching gas plasma 23, as shown in FIG. 14k, to form a phase shifter pattern 24. Finally, the remainder of the resist is incinerated out by an oxygen plasma 25, as illustrated in FIG. 14i. Through the foregoing steps, such a phase shift photomask containing phase shifters 24 as shown in FIG. 14m is completed.
According to the above-mentioned, conventional process for producing phase shift masks, wherein the transparent film 19 serving as a phase shifter has been patterned on the chromium pattern 17, it is required to draw a predetermined pattern on the resist layer 20 with the ionizing radiation exposure 21 and then develop, rinse, heat and descum to produce the resist pattern 22. It is further necessary to etch, either dry or wet, the resulting pattern after development to form the phase shifter pattern 24 and then remove the remainder of the resist. This results in an increase in the number of the production steps involved, making phase shifter pattern or other deficiencies likely to occur, and there is a rise in the production cost as well. If the number of the production steps is decreased, it will then be possible to prevent such deficiencies and achieve cost reductions.
Referring further to the above-mentioned, conventional process for producing phase shift reticles, how to correct usual reticles has already been established, should deficiencies be found prior to forming the phase shifters. However, how to correct the phase shifters of phase shift reticles has been unavailable as yet, thus presenting a problem that the phase shift reticles, even if in good shape, cannot possibly be put to practical use, should something wrong occur.